Linear-free piston Stirling engines (LFPSEs) may be employed to generate both heat and power for domestic use. The basic manner of operation of a dchp system incorporating such an LFPSE is detailed in various of our earlier published patent applications—see for example WO-A-03/076857.
LFPSEs typically comprise a displacer mounted via a flexible rod to displacer springs. Application of heat to the space above causes the displacer to reciprocate. A power piston is mounted coaxially and radially outwardly of the displacer flexible rod and reciprocates as well due to the gas forces acting between the displacer and power piston. Power output results from a linear alternator which comprises magnets mounted for movement with the power piston, relative to fixed windings.
It is highly undesirable (and potentially damaging) for moving parts of the LFPSE to come into contact with stationary, or other moving parts as a result of piston overstroke. Repeated collision of moving parts with stationary, or other moving parts will over time cause wear to the components, and apart from reducing the life of the engine, this process also results in small particles (arising from the collision wear) interfering with the narrow internal flow passageways of the engine.
In order to maximize the efficient use of energy input to the engine, it is usual to allow the LFPSE to operate at its mechanical resonant frequency. As a consequence, because the piston stroke is unconfined, the amplitude of reciprocation varies as a function of operating conditions and piston overstroke can occur. Various schemes have been proposed to address the problem of engine instability due, for example, to such piston overstroke. These schemes can broadly be categorised as detection and prevention.
Prevention techniques typically involve the use of magnets of opposing polarity to those in the alternator, to provide centering forces if the power piston starts to move outside of a safe range of movement. One such arrangement is described, for example, in U.S. Pat. No. 4,937,481. This technique suffers from the drawback that the magnet strength may reduce over time, reducing the effectiveness of the magnets as “stops”.
Detection techniques monitor engine parameters and usually provide for a rapid shutdown when it is determined that an instability such as piston overstroke is present or imminent. For example, it is known to employ vibration absorber proximity detectors with trip switches, as a safety-critical component. Piston overstroke is detected through determination of the relationship between the amplitude of reciprocation of the absorber mass and the piston stroke length.
In WO-A-2004/094860, an external mechanical or optical “on/off” switch is mounted on the vibration absorber of a Stirling engine, to ascertain when the amplitude of oscillation of the absorber (which is linked to the stroke length of the piston) exceeds a preset maximum. Upon detection of an overstroke condition, the heat to the Stirling engine is reduced to reduce, in turn, the piston stroke length.
In an alternative arrangement, as disclosed in JP-A-2003014322 (Sharp Corporation), an anti-collision mechanism for a Stirling refrigeration machine has a linear motor and the stroke of this is measured by determining the voltage and current applied to it. U.S. Pat. No. 5,836,165 (Hughes Electronics) suggests an arrangement which controls vibrations in a Stirling refrigeration machine by carrying out a Fourier analysis of the output signal of a vibration sensor.
Still a further scheme is disclosed in U.S. Pat. No. 6,536,326 (SunPower), wherein an acoustic measurement of the vibration is carried out using a microphone mounted upon the casing of the engine. The microphone output is used for feedback control of the piston in an LFPSE so as to back off piston amplitude when piston collision is detected. It will be understood from the foregoing that there are competing requirements on the engine in that, in many cases, it is desirable to run the engine at its maximum stroke (for maximum power output), but that at that maximum stroke, the possibility of overstroke is at its highest. As such, accurately predicting and/or detecting instability through piston overstroke is highly desirable. If engine shut-down is mandated upon detection of an overstroke, then false positives result in unnecessary engine shutdowns. These are at best an irritation (since shutdown stops any heat or power output). It is also necessary to wait for a significant period before restarting, whilst the engine burner cools below a threshold temperature, so as to avoid burner pre-ignition. Thus, unnecessary engine shutdowns are also time-consuming. False negatives are of course even more unacceptable since failure to prevent piston overstroke (through engine shutdown for example) can cause engine damage over time.
It is an object of the present invention to provide for improved techniques to address the problems of detection and/or prevention of engine instability.